An electrometer is an electronic circuit designed to measure very small currents (10−16 to 10−3 Ampere). Electrometers are commonly used to measure electrostatic charge on objects using a shielded sensing electrode such as a Faraday cup.
FIG. 1 illustrates the basic geometry, prior to taking a measurement, for measuring electrostatic charge with an electrometer 30 and a Faraday cup 32. The Faraday cup 32 is an apparatus with two electrodes designed to measure charge. The outer electrode 34 is a grounded electrode that electrically shields the inner sensing electrode 36. The sensing electrode 36 is preferably connected to an input terminal of the electrometer 30 using a shielded cable 38. The electrometer 30 has a high gain differential amplifier 40 with an integrating capacitor Cint connected between the output terminal VOUT and the negative input terminal. Prior to making a charge measurement, a “zero reset switch” 42 must be momentarily closed to insure that charge on Cint is zero. The high gain differential amplifier 40 maintains its two input terminals at the same voltage. The positive input terminal is connected to ground, so the negative input terminal is maintained at zero volts. Prior to taking a measurement, VOUT is zero because the negative input terminal is at ground potential and the voltage across Cint is zero.
FIG. 2 illustrates the electrometer 30 being used to measure the charge QX of an object 44 which has been placed in the Faraday cup 32. The charge QX induces an equal and opposite charge QI on the inner sensing electrode 36. This charge is drawn from the integrating capacitor Cint. The high gain differential amplifier 40 produces an output voltage VOUT proportional to QX, thereby accomplishing the measurement. The constant of proportionality or scale factor for the charge measurement is determined by the value of Cint, and QX may be determined as follows:QX=−VOUT·Cint 
Unfortunately, one difficulty with the measurement illustrated in FIG. 2 is that the high gain differential amplifier 40 has a fixed range of output voltages that is typically ±10 Volts. If Cint is too small, VOUT will exceed 10 Volts and the amplifier will saturate. With saturation, the measurement of QX is lost. To deal with this problem, commercially available electrometers commonly have several different integrating capacitors as illustrated in the electrometer 46 of FIG. 3. One of the capacitors Cint,1, Cint,2, Cint,3, Cint,4, Cint,5 is selected by a front panel switch 48 prior to making a measurement. Unfortunately, many commercially available electrometers operate in a manner that does not allow the device's measurement range to be changed once the measurement cycle has been commenced.
Since the measurement range of an electrometer being used to measure electrostatic charge is determined by the integrating capacitor that must be selected prior to a measurement, the range cannot be changed during the measurement because the charge on the integrating capacitor would have to transfer to the newly selected integrating capacitor without loss of charge. If the range selected is too sensitive, that is, the integrating capacitor selected is too small, the output of the high gain differential amplifier 40 will saturate and information on the charge being measured will be lost. If the measurement range selected is too big, that is, the integrating capacitor selected is too big, the instrument will lack the sensitivity to provide a useful reading. For example, assume that an operator selects a very big measurement range, 10−3 Coulombs/volt, utilizing a very large 1 mF capacitor. If the charge to be measured is 1 nC, the resulting output voltage will be only 0.000001 V. The high gain differential amplifier 40 would have to be very sensitive and the factor used to scale the voltage to determine the charge would have to be accurate to 6 significant figures. Calibration of sensitive equipment is, at best, 0.1% or 4 significant digits, which is a factor of 100× less than required.
Furthermore, electrometers used to measure charge need to be reset prior to making a measurement to insure that the charge stored on the integrating capacitor is zero. The time needed to discharge the integrating capacitor often exceeds 100 mS which is too long for many applications. Faster discharge times are desirable.
Furthermore, the components used in electrometers, such as the high gain differential amplifiers, may be susceptible to damage from electrostatic discharge (ESD) events or sparks. It would also be desirable to have an electrometer which was less susceptible to ESD damage.
Therefore, there is an need for a more versatile and efficient electrometer. In particular, it would be desirable to have an electrometer for use in electrostatic charge measurement which is capable of range adjustment during measurement, is simple and relatively inexpensive to manufacture, and which preferably has a shortened zero reset time and protection against electrostatic discharge.